More technical explanation (as I understood): The action should turn off Auto Position Update, provide a desired (not current) veloctiy vector for the Animator, so you can use it in a blend tree and update the Navmesh Agent Position once after it moves with root motion, so it matches the position of the character, for next frame calculation.

It looks like this is more-less possible using existing actions, except the last bit - updating the Agent position to the actual position of the character after it moves with root motion. It would be very useful to have it all in a "Root Motion Agent Move" Action.

No, no, that is the whole point of this Action. This empty slot is where you should assign a GameObject with Animator component using your Action It is empty by design. Anything that will be assigned to this track, should play the animation on this track.

How do you set the references in Animation tracks ("Bindings" section of the Director) in Timeline at runtime?

Because in my game the characters are spawned, they are not on the level until the game runs, so I can't reference them manually. Set Property Action doesn't give you access to "Bindings" section of the Playable Director.

I believe it's called PlayableDirector.SetGenericBinding, but I know absolutely nothing about coding.

Since this feature became a standard and the regular image effects became "legacy", we would use a set of Actions for changing the values (for example to smoothly change DOF at runtime, or gradually desaturate the scene, increase the noise and such).

Right now, with the "Set Property Action" you can only enable or disable chosen effect, but you don't have access to actual values.